The present invention relates to a noise elimination circuit for enabling pulsive noise-free reproduction of an audio signal from a record medium on which the audio signal has been recorded with frequency modulation.
In the past, when the audio signal is reproduced from the record medium on which the audio signal has been recorded with frequency modulation (FM), large amplitude noise is produced by the loss of reproduced signal due to dropout or by the discontinuity of the reproduced audio FM signal which may occur at the time of switching of playback tracks in a helical scan type magnetic tape recording and reproducing apparatus in which the audio signal is recorded in superposition on the video signal. This noise is eliminated by fully attenuating the output signal level of an audio signal amplifier during the dropout period.
FIGS. 1a to 1d show reproduced audio signal waveforms for explaining the operation of a prior art noise elimination circuit, in which FIG. 1a shows a reproduced audio FM signal waveform, FIG. 1b shows an FM-detected audio signal waveform, FIG. 1c shows a dropout detection signal and FIG. 1d shows an audio signal waveform with the noise being eliminated based on the dropout detection signal.
During the dropout period shown in FIG. 1a, large amplitude noise as shown in FIG. 1b is produced in the reproduced audio signal. On the other hand, the dropout detection signal as shown in FIG. 1c is produced by amplitude-detecting the reproduced audio FM signal (1a). Accordingly, by blocking the reproduced audio signal in response to the signal (1c), the noise-free audio signal as shown in FIG. 1d is produced. However, the method of merely cutting off the noise results in the discontinuity in the audio signal level. Accordingly, it can reduce the noise but cannot eliminate the noise to a practically acceptable level.